The invntion relates generally to the control of air vehicles at high angles of attack and more specifically concerns controlling the vortex patterns around forebodies to control the yaw and pitch during high angle of attack maneuvering of air vehicles, when conventional control surfaces become ineffective due to massive flow separation on lifting surfaces.
The current trend toward enhanced post-stall maneuverability of tactical aircraft demands control capability at angles of attack well above the range of conventional aerodynamic control surfaces. A pertinent example is the aft rudder, which rapidly loses effectiveness when immersed in the low-energy separated flow from the wing, requiring consideration of alternate means of yaw control.
An approach of considerable interest is to exploit well-known vortex asymmetry occurring on pointed forebodies at high angles of attack, which usually results in sizeable side forces acting well forward of the center of gravity. The prior art has investigated the use of air injection near the forebody nose to control the vortex asymmetry. An effective yaw power was demonstrated at high angles of attack on a particular forebody whose flattened cross-section favored the generation of strong vortices; the jet injection in this concept presumably acts as a fluidic switch to control the direction and degree of vortex asymmetry.
The question arises in the use of the jet injection technique whether a comparable level of yaw power would be obtainable when the prevailing forebody vortex system was not strong (e.g., on forebodies of shorter length or those having a circular or upright oval cross-section). The jet injection technique also was found somewhat limited in terms of the linearity control and effectiveness at large sideslip angles. In addition, to ensure jet fluid supply at high angles of attack when engine bleed air cannot be relied on the jet injection technique recommended a solid propellant blowing system. Such a system, however, has inherent control complexity, duration limitations, and servicing requirements.
Narrow, sharp-edge strakes have frequently been applied to pointed slender forebodies for modifying the high-alpha vortex shedding characteristics, as a solution to the zero-sideslip asymmetric problem as well as to improve lateral-directional stability. Unfortunately, fixed strakes have shown deleterious side effects such as high-alpha pitch up, diminished yaw damping, and marked reductions in dihedral effect.
It is an object of this invention to utilize a simple technique, exploiting the vortex asymmetry occurring on pointed forebodies at high angles of attack, for controlling yaw at hight angles of attack, that does not have the complexities and disadvantages of prior techniques.
Another object of this invention is to utilize a vortex technique for controlling pitch at high angles of attack.
A further object of this invention is to utilize deflectable strakes on the forebodies of air vehicles to control yaw and pitch at high angles of attack.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawings. For example, a symmetrical pair of strakes (later disclosed) alleviates the zero-sideslip force and yawing moment from the forebody at high angles of attack.